There are a large number of basic and applied researches on carbon nanotube field-effect transistor (CNT-FET) device. Still, a few ambiguous issues related to the transfer characteristics exist. One of these problems is that many CNT-FETs exhibit a hysteresis. Recent works have suggested that this hysteresis is due to a charge injection from CNT into surrounding dielectric. In this work, with a focus on the potential around CNT, we have directly imaged FET by means of scanning Kelvin probe microscopy (SKPM).
FET was fabricated by position-controlled CNT growth technique. We used the particles composed of alumina and iron as catalysts. CNTs were synthesized on a p-doped silicon wafer with thermally oxidized SiO2 (300nm) by chemical vapor deposition method. It is confirmed that the fabricated device is semiconducting FET and has large hysteresis of threshold voltage from the measurements of the transfer characteristics.
SKPM imaging was performed by a commercially available AFM. In order to measure surface potential during FET operation, a source electrode was electrically connected to bias voltage feedback electronics for SKPM measurement. The feedback dc- and the modulation ac-voltage (20 kHz, 2V) were applied to the sample.
The surface potential images of CNT-FET were taken on operative condition, which is that the drain voltage is fixed at -1 V and the gate voltage changes from -5V to 5V. We think that the tip has no influence on the transfer characteristics of FET since the monitored drain current is almost constant during imaging. When the negative (positive) voltage is applied to the back gate, the image contrast of SiO2 in the vicinity of CNT becomes brighter (darker) than that of the region without CNT. The area with difference in the contrast covered about 600 nm from the CNT. We think that the different contrast originates in the charges injected from the CNT channel into the surrounding dielectric because this feature cannot be seen in the image with no gate voltage. Furthermore, our conclusion is supported by the fact that the gate voltage does not alter the contrast in the vicinity of CNT fragment electrically isolated from the both electrodes.
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